This invention pertains to the hydroxy-alkylation of cellulose and more particularly to the use of a dimethyl sulfoxide/formaldehyde solvent mixture.
Hydroxyalkyl celluloses have been made for the most part via two-step processes employing aqueous alkali treatment prior to the alkylene oxide reaction. The alkali cellulose produced by interaction of caustic with cellulose, either in sheet or slurry form, is etherified with a 1,2-alkylene oxide in the presence of a diluent. In commercial practice the etherified product (slurry) is neutralized with acid and the resulting salts removed by a series of aqueous alcohol extractions. The final product is then dried.
This technique for the production of hydroxyalkyl cellulose is undesirable for a number of reasons. First, the strong alkali treatment of the cellulose which results in swelling and enhanced accessibility causes a substantial reduction in the cellulose chain length (degree of polymerization). Also the heterogeneous nature of the reaction between cellulose and alkylene oxide results in low oxide efficiency and non-uniform reaction of the cellulose hydroxyl groups. Finally, all the salts formed upon neutralization of the alkali must be removed together with other impurities (such as glycols and glycol ethers) which are by-products of reaction with the diluent.